Means and method for relaying frequency modulated signals



Patented Mar. 10, 1942 MEANS AND METHOD FOR RELAYING FREQUENCY MODULATED SIGNALS Edwin H. Armstrong, New York, N. Y. Application September 25, 1940, Serial No. 358,207

Claims.

The object of this inventon is to provide a relay system for frequency modulated signals, and particularly for a system in which the relayed or retransmitted signal is radiated at a slightly different frequency than the initial frequency.

The particular problem which is to be solved.

will appear from the following explanation. At

the present time there are in operation a number of frequency modulated transmitters using the wide band frequency modulation system described in my U. S. Patent #1,941,069 which have been assigned frequencies in the range of 42.5 to 43.5 megacycles. The various channels assigned are 200 k. c. in width, and hencethe frequency difference between the frequency of the signal which is to be received and the frequency which is to be retransmitted may be very small, requiring therefore highly selective receivers. In addition to the high selectivity of the receiver, it is usually necessary to adopt the further precaution of placing the receiver at some distance from the transmitting antenna to prevent various forms of interference from the very high signal lever encountered in the immediate field of the transmitter.

There are two types of relay transmitting systems. In one the received signal is detected and converted into its original form, and this current is used to modulate the second transmitter. In the other the received radio frequency current is heterodyned to the frequency to be transmitted, amplified up and transmitted without ever being converted into the audio or other signaling current. In each type of relaying certain difficulties are encountered. In the first type reception would be carried out with a superheterodyne receiver which can be readily designed to give the necessary selectivity to prevent interference from the transmitter. but which suffers from the fact that in the detection at the receiver and the remodulation of the local transmitter a certain amount of distortion is encountered. This, while hardly detectable in a single 'relay, eventually places a limit on the number of retransmissions which could be undertaken. In the second type reception would be accomplished without converting to audio, and the frequency to be transmitted would be produced by adding to, or subtracting from, the incoming frequency a locally produced current which might be of the order of 400 k. c. The desired frequency component would be selected out and transmitted and in this way the distortion incident to detection and modulation would be avoided. The circuits required to select the desired frequency for retransmission and the circuits required to protect the receiver from the effects of the transmitter would of necessity have to be extremely selective and consequently critical to adjust and maintain.

The present invention has for its object the provision of means whereby the difficulties of both types are overcome and the advantages ofboth retained. In accordance with the present method the incoming signal is heterodyned down to some relatively low intermediate frequency at which the desired selectivity may be obtained and then heterodyned up by means of a frequency which differs from the original heterodyning frequency by an amount equal to the difference between the received frequency and the frequency to be radiated. In order that changes in the frequency of the heterodyning currents used in the downward and upward conversions may not produce undesired drift in the radiated frequency from its assigned value, both of these heterodyning currents are obtained from the same oscillation source so that any drift in the frequency of one current occurs simultaneously in the frequency of the other and the net result is a cancellation of the drift. In my copending application for U. S. patent, Serial Number 313,496, filed January 12, 1940, I have described a method of accomplishing this result. The present arrangement affords some simplification over the method shown in the application previously filed.

The operation of the system will be readily understood from the following explanation.

Referring now to the drawings which form a part of this specification, Figs. 1 and 2 represent diagrammatically the complete relay system, in which the apparatus shown in Fig. 1 will usually be separated from the apparatus of Fig. 2 by a distance of some hundreds of feet, the apparatus represented in Fig. 2 being located at the transmitter. I

Referring now to the arrangement of Figs. 1 and 2 it has been assumed that a frequency of 42.8 megacycles is to be received and retransmitted as 43.4 megacycles. I represents the receiving antenna connected thru filter 2 to a balanced converter 3. This converter is supplied with a heterodyning current of 41.8 megacycles as shown. 4 represents a filter having a pass band sufficiently wide to pass 900-1100 k. c. current produced by the interaction of the incoming 42.8 megacycles signal with a k. c. deviation and the fixed frequency of 41.8 megacycles. 5, 6,

I and 8 represent selective amplifiers, 9 a limiter for the 1000 k. c. current, l0 a filter, ii an ampliher, and I2 a second filter connecting to a line I3.

It has been assumed that a distance of several hundred feet separates the arrangements of Figs.

1 and 2 which are connected by a transmission line l3 which terminates at l and which should preferably be a coaxial cable.

Referring now back to the heterodyning current of 41.8 megacycles, this is originally supplied over a line M from an oscillator at the second location at 10.45 megacycles through an amplifier MS, a doubler it which raises it to 20.9 megacycles and a second doubler H which supplies 41.8 megacycles current to the amplifier l8.

Referring now to Fig. 2, 90 represents the end of the line carrying the 1000 k. 0. intermediate frequency current, 2a a band pass filter and 211a second amplifier. .The output of this amplifier is fed into a balanced converter st. 22 is an oscillator preferably crystal controlled which generates a 10.45 megacycle current and excites an amplifier 23 which feeds the 10.45 mega'cycle current previously referred to over the line 30, Hi. A second amplifier 26 supplies current of the same frequency to a balanced converter 28 which is also supplied with current from a crystal oscillator 27 at a frequency of 600 k. c. The sum component of 11.05 megacycles is selected by a filter 29 and supplied to the balanced converter 3!, where it is combined with the 1000 k. 0. current from the amplifier ii. The sum of these two frequencies, 12.05 megacycles, is selected by the filter 32, amplified by the amplifier 33 and supplied to the converter 3% where it is combined with a 31.35 megacycle current from the amplifier 25. The sum of these two frequencies is selected by the tuned amplifier 35 to produce a current of 43.4 megacycles which is then amplified by the power amplifier 3t and transmitted from the antenna 37.

It will be observed from the foregoing explanation that if the transmitting frequency of the first station is maintained at its correct value, the transmitting frequency of the second station will likewise be correct since it is independent of any changes in any of the frequencies depending on the 10.45 megacycle oscillator and dependent only on the accuracy of the 600 k. c. oscillator.

Since this is such a small fraction of the value of the frequencieswith which we are here dealing, any variation which could normally occur'in it, is insignificant in the final result.

I have described what-I believe to be the best embodiments of my invention. I do not wish, however, to be confined to the embodiments shown, but what I desire to cover by Letters Patent is set forth in the appended claims.

I claim:

1. The method of receiving a frequency modulated signal wave of predetermined frequency and retransmitting the signal wave at a'slightly different frequency, which consists in receiving the wave, generating a low frequency and a higher harmonic of such low frequency, heterodyning the received wave down to an intermediate frequency by a heterodyning current of said higher quency to another frequency whose difference therefrom is equal to the frequency difference between the received and transmitted waves, raising the last-named frequency to the frequency to be transmitted by adding to it the intermediate frequency and an appropriate harmonic of said low frequency, and transmitting signal currents of the resulting frequency.

2. The method of receiving a frequency modulated signal wave of predetermined frequency I and retransmitting the signal wave at a slightly difierent frequency, which consists in receiving the wave, generating a low frequency and a higher harmonic of such low frequency, heterodyning the received wave by a heterodyning current of said higher harmonic frequency to produce currents of intermediate frequency, converting the said low frequency to another frequency whose difference therefrom is equal to the frequency difference between the received and transmitted waves, beating the last named frequency with the intermediate frequency to produce a higher frequency, beating said higher frequency with a harmonic of said low frequency and transmittingPsignal currents of the resulting frequency.

3. Means for receiving a frequency modulated signal wave of predetermined wavelength and for relaying said wave on a slightly different wavelength comprising, in combination, a converting device, means for energizing said converting device by the received wave. a low frequency oscillator, frequency multiplying means coupled to said oscillator for generating a harmonic of the oscillator frequency, means connecting said frequency multiplying means to the converting device whereby currents are generated therein of a low intermediate frequency, a second converting device, means for supplying the low intermediate frequency currents to the second converting de-* vice, means connecting the low frequency oscillator with the second converting device for supplying thereto a frequency differing from that of the low frequency oscillator by the difference in frequencies betweenthe received and transmitted waves, means connected to the output of the second converting device and to the low frequency oscillator for generating currents of the wave length to be transmitted and means for trans mitting the resulting currents.

4. A signal relaying means as set forth in claim 3 in which the means connected to the output of the second converting device comprises means for multiplying the frequency generated by the low frequency oscillator.

5. A signal relaying means as set forth in. v I

claim 3 in which the means connecting the low frequency oscillator with the second convertin device includes a second low frequency oscillator arranged to generate a substantially constant EDWIN H. ARMSTRONG. 

